Brushless DC motors typically comprise a magnetic rotor and one or more stator coils. For driving the rotor by applying a suitable driving current waveform to one or more stator coils it is important to know the rotor position in relation to the stator coils. In function of this position the driver generates a current in a specific direction through a specific coil to generate torque so as to turn the rotor in a desired direction. When the rotor has turned beyond a certain position, (the commutation point), the current direction needs to be inverted (commutated) so that it is again in the appropriate direction to generate torque in the desired direction.
Methods and systems for controlling the commutation of a brushless DC motor may be based on Hall sensors. These Hall sensor(s) detect the position of the rotor in relation to the stator coils and based thereon the current through the motor coils(s) is controlled.
US2016118916 discloses a motor driving apparatus which comprises a Hall sensor for obtaining a position of the rotor and a current monitoring circuit for asserting a zero current detection signal when a reversion of direction of a coil current flowing into the motor coil is detected. Using this configuration, it is possible to detect the amount of phase lead and lag based on the timing of the zero current and to determine an appropriate correction amount based on the detected amount of phase lead and lag. Based on the correction amount the transition sequence of the H bridge circuit is shifted forward or backward in time.
In order to avoid the need for a Hall sensor, sensorless commutation methods are developed. Such sensorless methods may for example monitor the BEMF (back electromotive force) voltage for estimating the position of the rotor. Sensorless methods make the motor construction less complex, because the hall sensor position is critical for the operation of Hall based commutation. In low-cost high volume fan systems such as they are used for CPU cooling, refrigerator ventilation, power converter cooling, etcetera, but also in low cost pumps to displace liquids, single coil fans, based on hall sensing are applied. In case in such low-cost systems the hall sensor could be avoided, it is clear, that the single coil motor controller may no longer have to be applied close to the rotor, or even not inside the motor assembly, for instance a fan or pump, anymore. In current low-cost systems remote controllers typically use PWM input signals, and FG/RD output pins, as communication interface to control the fandrivers which are integrated into the remote fan. In case of sensorless control, a significant system simplification can be achieved by locating the fandriver close to the controller, or even integrate into the remote controller.
Another problem is that the BEMF voltage can only be measured correctly, if there is no current flowing in the coil. For this purpose, a window with no current in the coil must be created in the driving wave form profile. In case of single coil motor control such interruption of the phase current might introduce a torque ripple in the torque generated by the motor, causing audible or EMC noise.
In three phase BLDC motors, a well-known first commutation strategy, referred to as trapezoidal control is to monitor the BEMF voltage zero crossing (BEMF_ZC) in the third coil which is not driven, while delivering the torque by driving the first and second coil.
In more advanced three phase BLDC control strategies, referred to as sinewave strategies, the commutation is defined while all three coils are driven. There exist methods wherein at predefined moments every 60 degree or multiples of 60 degree, the rotor position is defined.
In even more advanced methods, referred to as Field Orient Control (FOC), the current is continuously monitored.
As the methods get more complex, the needed calculation increases drastically. For FOC control 8-bit, 16 bit and even 32 bit CPUs are applied. Also the performance of these methods is strongly depending on the motor magnetic design. In all cases the delivering of constant torque requires a continuous flow of a controlled amount of current. In typical cases the motor magnetic design is not optimal. The control of the amount of current has to compensate such motor deficiencies, leading to further drive complexity.
In all sinewave methods, the essential part is to smoothly transfer the torque vector from one coil to the next with minimum torque ripple. In single coil BLDC control, such smooth transfer is not possible, because the torque has to go through zero at the point where rotor north pole transits to a south pole.
In a single coil motor the trapezoidal method cannot be applied because there is no undriven coil, also FOC methods are not obvious because of the strong non-linear nature of the single coil fan torque every 180 electrical degrees.
Moreover, sensored trapezoidal three phase solutions are using three hall sensors, which are spaced 60 degree or 120 degrees apart. This allows to ensure the startup of the fan occurs in the wanted direction. Similar for single coil fans a single hall element or hall sensor is applied. Since no spacing between multiple hall sensors is required, such hall sensor can easily be integrated into the motor controller. The startup direction of such single coil motors is typically ensured by magnetic design of the motor, in which the reluctance zero point is slightly offset by adjusting the stator shoe design. Intelligent commutation methods for single coil motors, referred to as soft switching, require speed information of the fan rotation, in order to smoothly control the transition of the current. At start up no speed information is available. Therefore at startup of a single coil BLDC motor the driving methods for driving a single coil brushless DC motor may be subdivided, as illustrate in FIG. 1, in 3 steps. First the position of the rotor is detected, next the rotor is accelerated, and finally the motordriver enters a steady state operation mode. The steady state operation usually has the highest performance requirements, for instance lowest noise, highest speed, highest efficiency. While during position detection and acceleration, some loss of performance may be acceptable in exchange for increased robustness. During steady state operation some robustness may be exchanged to achieve maximum performance.
The left flowchart of FIG. 1 shows a possible driving method in case of hall-sensor based prior art motor drivers. The rotor position is detected by the hall sensor and next the rotor is accelerated according to a startup procedure. The startup procedure can be left, for instance as soon as the hall sensor has toggled at least once. In some advanced fandrivers multiple start up procedures or acceleration procedures may exist, which each have their exit criteria. At one point in time the steady state operation mode is entered, in which the hall sensor signal is used to define the timings for controlling the commutations, in a way which is optimized for steady state performance, such as for instance low noise operation, high efficiency, etc. Some advanced prior art hall-sensor based single coil motor drivers apply predictive control, in which the commutation procedure is initiated some time prior to the BEMF_ZC. This allows to optimize the energization wave forms for acoustic noise, robustness, maximum torque or any combination. Also for sensorless motor drivers such three steps can be distinguished in which the hall sensor input is replaced by an alternate method, as illustrated in the right flow chart of FIG. 1.
Some prior art single coil motor drivers start up by applying a pulse width modulated driving signal with an output duty cycle DCout=100%, until the hall sensor has toggled at least one time. If the hall sensor does not toggle within a time Ton, the fandriver enters LRP (Locked Rotor Protection). After a time Toff, the fandriver retries to start. Typically, the ratio Ton/Toff is between ⅕ and 1/10.
A disadvantage of such start up method, is that an excessive inrush current may be drawn, especially for low ohmic coil resistance applications. Such excessive peak currents may also lead to audible noise when the fan starts up.
Therefore, in more advanced prior art motor drivers, different energization wave forms are applied, which are referred to as soft start waveforms.
FIG. 2 shows a typical example of a startup energization wave form for a hall sensor based motor driver, in which the duty cycle output DCout is progressively increasing from 50%. If the motor driver hall sensor toggles three times, the motor driver will adjust its output duty cycle DCout from its actual value to the requested value by the PWM input. Two examples are given in the left picture, in which the PWMinput is either 80%, or 10%.
In the right image the situation is shown in case of Locked Rotor Position (LRP). DCout increases until 100% at, and then remains flat until Ton. It is clear for the technical expert that many variants on such energization wave form can be realized.
Also during initial acceleration, or in case of sudden load changes, predictive algorithms which rely on previous timing information, are vulnerable. Therefore, they should be complemented by robust driving methods, which allow to converge towards and seamlessly enter into more high-performing predictive driving methods.
Therefore, there is need for controlling single coil brushless DC motors in a sensorless way which are more robust as complement to high-performing sensorless methods during non-steady state operation of the motor.